Among various types of photosensor circuits that implement individual pixels of MOS-type image sensors are a type presenting linear output characteristics in response to variation in intensity (or illuminance) of incident light, and a type presenting logarithmic output characteristics in response to variation in intensity of incident light. The following paragraphs outline these types of photosensor circuits and explain their characteristics in terms of an S/N ratio, dynamic range, afterimage, sensitivity in a low-intensity light condition, etc.
FIG. 16 is a diagram showing an example construction of the photosensor circuit presenting linear output characteristics. The photosensor circuit 101 of FIG. 16 includes a photodiode PD provided as a photosensor element (i.e., photoelectric conversion element) for detecting incident light (light signal) L1 and converting the detected incident light L1 into an electrical signal. The photodiode PD includes a capacitor C1 that is implemented with parasitic capacitance (including floating or stray capacitance of wiring). The photosensor circuit 101 further includes a MOS transistor Q1 for charging and discharging the capacitor C1, a MOS transistor Q2 for amplifying a terminal voltage of the capacitor C1, and a MOS transistor Q3 for selectively outputting the amplified terminal voltage (Vout) as a pixel signal. Resistor R is connected to a drain terminal of the MOS transistor Q3.
Given voltages V1 and V2 are applied, by a voltage controller 102, to a gate terminal G1 and drain terminal D1, respectively, of the MOS transistor Q1. Further, given voltages V3 and V4 are applied, by the voltage controller 102 (and a pixel selection circuit etc.), to a gate terminal G3 of the MOS transistor Q3 and outer terminal T1 of the resistor R, respectively. Generation timing at which the voltages V1-V4 are generated via the voltage controller 102 is instructed by a timing signal generation section 103.
The photosensor circuit 101 behaves as follows. The gate voltage V1 of the MOS transistor Q1 is switched to a high level at predetermined initialization timing with the drain voltage V2 of the MOS transistor Q1 kept at a high level, so that an electrical charge remaining in the capacitor C1 of the photodiode PD is discharged to the drain of the MOS transistor Q1. Then, the gate voltage V1 is switched to a low level (0 volt) to turn off the MOS transistor Q1. After that, the capacitor C1 of the photodiode PD is caused to store an electrical charge. Terminal voltage produced in the capacitor C1 by the electrical charge storage is applied to a gate of the transistor Q2. Then, once the MOS transistor Q3 is turned on upon lapse of a predetermined light exposure time in the photodiode PD, a light signal is output, as the voltage Vout, from a drain of the MOS transistor Q3.
In the aforementioned photosensor circuit 101, a photoelectric current flowing through the photodiode PD is controlled by a discharge current of the charge stored in the capacitor C1 of the photodiode PD. Thus, the output voltage Vout, which is a sensor output of the photosensor circuit 101, presents linear output characteristics proportional to the discharge current. Because the photosensor circuit 101 arranged in the aforementioned manner can control the sensor output on the basis of the light exposure time, it can function as part of a storage-type image sensor. However, because the output voltage Vout is proportional to the intensity of the incident light L1, it would get saturated when strong-intensity light is input, and thus, the photosensor circuit 101 can not achieve a wide dynamic range.
Photosensor circuit similar in construction to the aforementioned photosensor circuit 101 is disclosed, for example, in JP-2000-329616 A.
FIG. 17 is a diagram showing an example construction of the photosensor circuit of the type presenting logarithmic output characteristics. In FIG. 17, elements substantially identical to the above-described elements of FIG. 16 are indicated by the same reference characters as in FIG. 16 and will not be detailed to avoid unnecessary duplication. The photosensor circuit 201 of FIG. 17 includes a MOS transistor Q21 in place of the MOS transistor Q1, and the MOS transistor Q21 has a gate electrically connected to its drain. Photodiode PD, capacitor C1, MOS transistors Q2 and Q3, resistor R and other circuit elements are identical to those described above in relation to FIG. 16. In the photosensor circuit 201, a sensor current of the photodiode PD is converted, by the MOS transistor Q21, into a sensor voltage having logarithmic characteristics in a weak inversion state.
In the photosensor circuit 201, the gate of the MOS transistor Q21 is electrically connected to its drain as noted above so that a drain voltage and gate voltage are set at a same predetermined voltage level, and the MOS transistor Q3 is turned on so that the light signal is detected as the output voltage Vout. High-level gate voltage is supplied, by the voltage controller 102, to the gate terminal G3 of the MOS transistor Q3.
The photosensor circuit 201 can achieve a wide dynamic range by virtue of its logarithmic output characteristics. However, with the photosensor circuit 201, where a photoelectric current flows via the channel of the MOS transistor Q21, it is not possible to improve the S/N ratio by increasing the light exposure time as achieved by the storage-type image sensor. Therefore, the photosensor circuit 201 would present a poorer sensitivity in a low-intensity light condition than the storage-type image sensor implemented by the aforementioned photosensor circuit 101. Further, when the current flowing to the MOS transistor Q21 is of a low level, the photosensor circuit 201 tends to produce an unwanted afterimage because impedance of the channel of the MOS transistor Q21 increases.
Example of the photosensor circuit of the presenting logarithmic output characteristics is disclosed in JP-2000-329616 A.
As noted above, the photosensor circuit of the type presenting linear output characteristics can not achieve a wide dynamic range because its detection signal is proportional to the intensity of incident light and gets saturated when strong light is input. Further, the photosensor circuit of the type presenting logarithmic output characteristics presents a poor sensitivity in a low-intensity light condition, and, when the current flowing to the MOS transistor Q21 is of a low level, it tends to produce an afterimage due to an increased impedance of the channel of the MOS transistor Q21.